1.4 The origin of the vertebrates
Vertebrates such as ourselves are by definition animals with a backbone (or vertebral column, paired limbs, a skull and various other structures. Until recently vertebrates were thought to extend back only into late Ordovician times, some 450 million years ago. At this time fossils of strange-looking fish with bony headshields, such as Sacabambaspis (Atlas, pp. 70-71), appear in the fossil record. These jawless fish (called agnathans) are only very distantly related to the sole living agnathans, the lampreys and hagfish. However, these Ordovician creatures are already highly evolved and clearly had yet more ancient ancestors.
To understand what such ancestors might have been like we need to consider the backbone - the fundamental vertebrate feature - in a bit more detail. The precursor to the backbone is a stiffening rod, called the notochord, details of which we know from the study of developing vertebrate embryos and the few surviving animals which have retained a notochord, such as the lancelet Branchiostoma.
Before going any further, click on 'View document' below and read pages 66-67 from Douglas Palmer's Atlas of the Prehistoric World.
What is the main function of the notochord?
It lengthens and stiffens the body, allowing the blocks of muscles to flex the body sideways into zigzag bends for swimming (Atlas, p. 66).
Why is Pikaia classified as a chordate but not a vertebrate?
It has a notochord but not a backbone, nor other vertebrate features such as paired limbs (Atlas, pp. 66-67).
The extinct conodonts (late Cambrian to end Triassic in age) have been shown to possess not only a notochord but also tooth structures and paired eyes, which seem to suggest that they were more advanced than chordates such as Pikaia and close to the earliest vertebrates. (You will read about conodonts shortly on pp. 70 of the Atlas.) Some early Cambrian fossils recently found in China are thought to preserve all the basic chordate features plus some more advanced vertebrate ones, namely gills as well as paired eyes. If so, then it is clear that some animals close to the vertebrate bodyplan had already appeared by early Cambrian times, and chordate ancestry probably reaches back into the late Precambrian.
Humans belong to the vertebrate group known as mammals. Using Section 1.3, place mammals and the other four main vertebrate groups in order of their evolutionary appearance.
The earliest generally recognised group of vertebrates to appear in the fossil record are fish, followed by amphibians, reptiles, mammals and finally birds.